Quoting Dennis King <[log in to unmask]>:
> Charles scríbas:
> > > the vowel of the first syllable of "Beltaine" (modern "Bealtaine",
> > > Sc.G. "Bealltainn") is historically short
> > May-day, Irish béalteine, Early Irish beltene, belltaine
> > Béaltaine (Meath) and Beáltaine (Connacht and Donegal)
> I'm not too surprised to see long vowels in modern variants.
> Early spellings with a long 'é' (*Bél(l)taine) are lacking,
> however, as far as I know. Since only the historical quantity
> of the vowel is relevant to the etymology, "bél" (mouth) still
> seems to be excluded.
DIL also says that "Bil" and Biel" (Corm. Y 122 O'Mulc. 150; ZCP iii 246.4)
can be used which are said to mean "mouth" (in one of its variations).